I don't think you need a gateway on eth0 - a gateway is "if nothing else matches send the request to this IP" thing, so it should generally be the IP of your DSL modem, and be configured on the interface which is physically connected to the modem. Apart from that, it's not quite clear what you're trying to achieve and what the problems are. "nothing works at all" is such a wonderful description...
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SergeyJan 11 '13 at 23:37

@Sergey eth0 is connected to the switch. You should take a look again because everything is explained.
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Sein OxygenJan 12 '13 at 0:03

I'd suggest to split this question into at least 3 separate questions and solve the problem in steps: 1. properly configure network adapters (see my previous comment) so you can access internet from this machine; 2. Configure NAT so other machines connected to the switch can access internet via this machine while configured manually; 3. Configure DHCP; 4. deal with network boot. You'll find that many questions are already answered on this site or elsewhere in details.
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SergeyJan 12 '13 at 0:21

Although your answer is 100% correct, it might also become 100% useless if that link is moved, changed, merged into another one or the main site just disappears... :-( Therefore, please edit your answer, and copy the relevant steps from the link into your answer, thereby guaranteeing your answer for 100% of the lifetime of this site! ;-) You can always leave the link in at the bottom of your answer as a source for your material...
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FabbyJul 8 at 13:10